U.S. Pat. No. 4,953,538 dated May 29, 1987 discloses a "piggy back" evaporator which is constructed to "increase evaporation through the utilization of waste heat." The piggy back pan has a closely corrugated bottom panel, includes a sealed exhaust hood with an exhaust pipe and a blower which supplies air under pressure to a plenum 44 connected to a plurality of down pipes 48. Each down pipe is, in turn, connected to an elongated pipe for supplying compressed air into each grooved portion of the corrugated piggy back pan. An inclined collector sheet extends substantially across the entire area of the piggy back pan to receive and collect the steam condensate which in the form of water droplets falls onto the upper surface of the collector sheet and runs downwardly on the sheet into a drain trough. This collector sheet construction impedes the direct upward flow of steam emitted from the flue pan and since it cannot rise vertically to contact the underside of the piggy back pan, the steam follows a more circuitous route about the lower end of the collector sheet, as illustrated in FIG. 3. Consequently, some steam will condense on the underside of the collector sheet and a portion will condense on the upper surface of the collector sheet where the steam is flowing upwardly counter to the direction of the condensate runoff on the upper surface of the collector sheet. This arrangement results in two distinct disadvantages. First, a substantial portion of the condensate will drain back into the flue pan which, in essence, defeats the basic purpose of the apparatus, i.e., to reduce the water content of the maple sap. Secondly, as the steam flows over the upper side of the collector sheet, it is cooled by the counter flowing condensate which thereby reduces the amount of latent heat transferred from the steam to the sap and accordingly reduces the efficiency of the apparatus.
It is the principal object of this invention to provide an auxiliary evaporator pan disposed in superposed relation on the flue pan of conventional construction such that it is operable at higher efficiencies than conventional flue pans and which can be manufactured at lower cost than the piggy-back pan of the prior art.
The above and other objects and advantages of this invention will be more readily apparent from a reading of the following description taken in conjunction with the following drawings in which: